Abstract

A previously developed technology of laser-generated stress pulses has been adapted to measure the tensile strengths of interfaces between composite thermal barrier coatings of alumina and zirconia, and steel substrates. In the basic technique developed previously (Gupta et al., 1994), a compressive stress pulse is generated on the back side of the substrate disc by exfoliating a constrained metallic film via the impingement of a Nd:YAG laser pulse. This compressive stress pulse propagates through the substrate and reflects into a tensile wave from the free surface of the coating that is deposited on its front surface. The returning tensile pulse pries off the interface if its amplitude is sufficiently high. The interface stress is determined by recording the coating’s free-surface velocity by using a laser displacement interferometer. Because of the short rise time of the stress pulse, an interfacial region of approximately 70 to 150 micrometers is uniformly stressed. This results in the failure of the weakest link in the region which is spanned by the coating, interface and the substrate material. In addition, such a short pulse is able to invoke a rather local response from the interface such that minute structural and chemical changes are directly reflected in the measured strengths.

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